Sassen-1999-Thermogenic gas hydrates and hydro

OrganicGeochemistry30(1999)485±497

Thermogenicgashydratesandhydrocarbongasesincomplexchemosyntheticcommunities,GulfofMexicocontinentalslope

RogerSassena,*,SamanthaJoyeb,StephenT.Sweeta,DebraA.DeFreitasa,AlexeiV.Milkova,IanR.MacDonalda

aGeochemicalandEnvironmentalResearchGroup,TexasA&MUniversity,CollegeStation,TX77845,USAbDepartmentofMarineSciences,UniversityofGeorgia,Athens,GA30506,USA

Received17November1998;accepted16March1999(Returnedtoauthorforrevision13January1999)

Abstract

Whereabundantattheseaˉoor,thermogenicgashydratesimpactbacterially-mediatedprocessesinchemosyntheticcommunitiesdependentonmethaneandH2S.OurmaingashydratesitesareatH540mwaterdepthandrelativelylowtemperature(H78C).Gashydratesoutcropasvein-?llingsinhemipelagicmudsneargasventswithinchemosyntheticcommunities.Molecularandisotopicpropertiesofhydrate-formingC1±C5hydrocarbonsandCO2provideinsighttobacterially-mediatedprocesses.Hydrate-boundmethaneisalteredbybacterialoxidation,asindicatedbyenrichmentof13Canddeuterium(D),andbyCO2depletedin13C.Thedegreeofgashydratealterationappearsrelatedtodurationofexposureattheseaˉoor.Inhydrate-associatedsediments,bacterialoxidationofamixedpoolofhydrocarbonsyieldsanetproductionofCO2depletedin13C.Bacterialoxidationofhydrate-boundmethaneandfreehydrocarbongasesinadjacentsedimentscouldcontributetogashydratedecomposition.Somethermogeniccarboninsedimentscouldberecycledviamethanogenesistoyieldanetproductionofbacterialmethanedepletedin13C.Ourresultsstrengthenthehypothesisthatgashydratescouldfavorlifeinotherextremeenvironmentsatlowtemperatures.#1999ElsevierScienceLtd.Allrightsreserved.

Keywords:Gashydrate;Hydrocarbons;d13CanddDofmethane;Chemosyntheticcommunities;GulfofMexico

1.Introduction

ComplexchemosyntheticcommunitiesontheGulfofMexicocontinentalslopewere?rstdiscoveredduringtrawlinginareascharacterizedbysedimentscontainingfreehydrocarbongases,gashydrates,bac-teriallyoxidizedoilrichintoxicaromatichydrocar-

*Correspondingauthor.Fax:+1-409-862-2361.E-mailaddress:sassen@gerg.tamu.edu(R.Sassen)bons,andauthigeniccarbonaterockatwaterdepthsinthe600±700mrange(Kennicuttetal.,1985).Theselow-temperaturecomplexchemosyntheticcommunities(tubeworms,methanotrophicmussels,clams,andvar-iousotherfauna)deriveenergyfromreducedcarbon,mainlymethane,andbacterialH2S.Laterresearchcon?rmedthatanumberofcomplexchemosyntheticcommunitieswerespatiallyassociatedwithgashydrateontheGulfslope(Sassenetal.,1993;MacDonaldetal.,1994;Sassenetal.,1994;SassenandMacDonald,1994,1997;Fisheretal.,1998;Sassenetal.,1998).0146-6380/99/$-seefrontmatter#1999ElsevierScienceLtd.Allrightsreserved.PII:S0146-6380(99)00050-

486R.Sassenetal./OrganicGeochemistry30(1999)485±497

Fig.1.LocationmapshowingstudysitesandJollietField(GreenCanyonprotractionarea)withinabeltofshallowgashydrates,seeps,chemosyntheticcommunities,anddeepoilandgasaccumulationsfromGardenBankstoMississippiCanyon.

Becauseoftheenormousglobalabundanceofmethanegashydrates(Kvenvolden,1993),evidenceofalinkbetweengashydratesandchemosyntheticcommunitiesissigni?cant.Deep-seagashydratesareice-likecrystallinesub-stances(minerals)inwhichhydrocarbonandnon-hy-drocarbongasesofspeci?cmoleculardiametersareheldbyhydrogenbondingwithinrigidcagesofwatermolecules.StructureIhydrate(bodycenteredcubiclattice)generallycontainsmethaneandothernon-hy-drocarbongasesformedbybacterialactivityatshallowdepthsinsediment(Sloan,1990;Kvenvolden,1993,1995).Incontrast,othergashydratescontainthermo-genicgasesthathavemigratedtoshallowsedimentsfromdeephotsubsurfacehydrocarbonsystems.StructureIIgashydrate(diamondlattice)encagesC1±C4hydrocarbons,andstructureHhydrate(hexagonallattice)encagesC1±C5hydrocarbonstoincludei-C5(Sloan,1990).ThermogenichydratesalsocontainCO2andothernon-hydrocarbongasesformedinthedeepsubsurface(Brooksetal.,1986).GashydrateswithallthreecrystalstructuresoccurontheGulfofMexicocontinentalslope(Kvenvolden,1995;Boothetal.,1996;SassenandMacDonald,1994,1997).ThermogenicgashydratesoutcropontheGulfofMexicoslopeseaˉoor(MacDonaldetal.,1994).Incontrasttogashydrateburiedattensorhundredsofmetersinsediments(Kvenvolden,1995),outcroppinggashydratesoftheGulfslopeexistinarelativelyun-stableenvironment.ShallowgashydratesaresubjecttorapidtemperaturechangesfromperiodicinvasionsofwarmloopcurrentwaterontheGulfslope(MacDonaldetal.,1994).RatesofgasventingontheGulfslopeˉuctuateepisodicallyoverbothlongandshorttimescalesbecauseofchangesinsealevel,andbecauseofdynamicgeologicresponsestorapidsedi-mentloading(RobertsandCarney,1997).Fluctuationsinequilibriumconditionscoulda??ect

gas

R.Sassenetal./OrganicGeochemistry30(1999)485±497487

Fig.2.Sketchoftypicalgashydratemound(H1±3macross)modeledaftertheBushHillsiteincludingsamplingstationsBHHYD-1(gashydrateandmoundsedimentcover)andBHST-2(nearbytubewormsediments).FromphotographinSassenetal.(1998).

hydratesandchemosyntheticcommunities(Carney,1994).Recentresearchsuggestedthatbacterialalterationofsolidgashydrateoccursviatheoxidationofmethane(Sassenetal.,1998).Here,weinvestigatethemolecularandisotopicpropertiesofC1±C5gasesandCO2inthelargergashydrateenvironmenttoprovideinsighttobacterially-mediatedgeochemicalprocessesthatimpactcomplexchemosyntheticcommunities.Findingsofthisstudyexpandthehypothesisthatabundantsea-ˉoorthermogenicgashydratescoulda??ectthestabilityofecosystemsinextremeenvironmentsatlow-tempera-tures(Carney,1994;Sassenetal.,1994,1998).

2.Geologicandgeochemicalsetting

OurGreenCanyonstudyareaispartofalargebeltspanningtheGulfofMexicoslope(Fig.1)thatcon-tainssea-ˉoorgasvents,oilseeps,gashydrates,che-mosyntheticcommunities,andsubsurfaceoilandgas?elds(RobertsandAharon,1994;Sassenetal.,1994;MacDonaldetal.,1996).ThemostrecentepisodeofhydrocarbonventingappearstohaveinitiatedacrossGreenCanyonduringthelatePleistocene(Aharonetal.,1997).GeochemicalcorrelationofoilandgasfromourGreenCanyonstudyareaso??shoreLouisianaestablishesalinktoadeeplyburiedMesozoichydro-carbonsystem(6±10km)thatgaverisetoshallower(2±3km)oilandgasaccumulations(Kennicuttetal.,1988).VerticalmigrationofˉuidsinGreenCanyonisfacilitatedbyactivelymovingsaltbodiesandfaults,resultinginrapidˉuidˉowandventingontheseaˉoor.TheBushHillstudysiteisafault-relatedseepfea-tureinGreenCanyon(GC)Block185(27847.5'Nand91830.5'W)atabout540mwaterdepth(Fig.1).ThefaultsarerelatedtothesubsurfacehydrocarbonsystemthatgaverisetonearbyJollietFieldonGC184(Reillyetal.,1996).Meansea-bottomwatertempera-tureisH78C(MacDonaldetal.,1994),andthepress-ureisH5400kPa.CopiousstreamsofthermogenicgasventcontinuouslytothewatercolumnatBushHill(MacDonaldetal.,1994;SassenandMacDonald,1997).Sea-ˉoormounds,H1±3macross,havegashydrateoutcroppingontheirˉanks.Gashydrateappearstorapidlycrystallizeduringventingasvein?llingswithinhemipelagicmud.Vein-?llinggashydrateistypicalinsettingswithrapidˉuidˉow(GinsburgandSoloviev,1994;Ginsburg,1998).GashydratemoundsatBushHillhavepersistedatthesamelocationsfor>5years(MacDonaldet

al.,

488R.Sassenetal./OrganicGeochemistry30(1999)485±497

Fig.3.ExposedstructureIIgashydrateatˉankofgashydratemoundatstationBHHYD-1,BushHillsite.Notescalebar.1994),andarethuslikelytobeimpactedbyexposureattheseaˉoor.Thinlayersofhemipelagicmud(H10±30cm)caphydratemoundsatBushHill.Sedimentsaretypicallybioturbatedbymacro-faunaandcontainaromatic-richbacteriallyoxidizedcrudeoil,freegas,dispersedgashydratenodules,authigeniccarbonaterock,H2S,pyrite,andelementalsulfur(Sassenetal.,1993,1994).Authigeniccarbonatedepletedin13Coccursasliningsoftubularpolychaeteburrowswhichextenddowntothegashydratesurface(Sassenetal.,1998).AcomplexchemosyntheticcommunityinhabitstheBushHillsite(MacDonaldetal.,1989).Speci?cally,whiteororangeBeggiatoamats(Larkinetal.,1994)occupythesediment±waterinterfaceongashydratemounds;vestimentiferantubeworms(mainlyLamellibrachian.sp.)areadjacenttothemounds(Fig.2).TheGC234studysite(27844.8'Nand91813.3'W)isafault-relatedseepareaovershallowsalt(Reillyetal.,1996)atH543mwaterdepth(Fig.1).Meantem-peraturesandpressuresaresimilartothoseobservedattheBushHillsite.Gashydratevein-?llingsout-croppedonanupthrownfaultscarp.Whensampledin1997,thegashydratewasdecomposing,asevidencedbycontinuousformationandreleaseoffreegasbubblesfromitssurface.Gashydrateinstabilityresultedinundercuttingofthescarpface,andby1998thefeaturewasabsent.WeassumetheGC234gashydratewasmorerecentlyexposedthangashydrateattheBushHillsite,andwasatransientfeature.Hydrocarbongeochemistry(Brooksetal.,1986,Sassenetal.,1994),andchemosyntheticfauna(MacDonaldetal.,1990a),aregenerallysimilartothoseofBushHill.Fisheretal.(1998)observedunidenti?edpolychaeteworms(``iceworms'')thatclustereddirectlyonout-croppinggashydrateattheGC234studysite.Thebrinepoolstudysite(GC233:27843.4'Nand91816.8'W)isananoxicbrinepoolatawaterdepthofabout640m(Fig.1).Thebrinepoolarisesfromˉuidˉowalongasalt-relatedfault(Reillyetal.,1996).Thebrinepoolisrimmedbymethanotrophicmussels,whereastubewormsandotherchemosyntheticfaunaarepresentinlowabundance.Dissolvedmethaneofbacterialoriginsaturatesthebrineitself,andfree